Article pubs.acs.org/JPCA Thermal Decomposition Mechanism for Ethanethiol † † † ‡ † † AnGayle K. Vasiliou,*, Daniel E. Anderson, Thomas W. Cowell, Jessica Kong, , William F. Melhado, † † Margaret D. Phillips, and Jared C. Whitman † Department of Chemistry and Biochemistry, Middlebury College, Middlebury, Vermont 05753, United States ABSTRACT: The thermal decomposition of ethanethiol was studied using a 1 mm × 2 cm pulsed Δ → silicon carbide microtubular reactor, CH3CH2SH + Products. Unlike previous studies these experiments were able to identify the initial ethanethiol decomposition products. Ethanethiol was entrained in either an Ar or a He carrier gas, passed through a heated (300−1700 K) SiC microtubular reactor (roughly ≤100 μs residence time) and exited into a vacuum chamber. Within one reactor diameter the gas cools to less than 50 K rotationally, and all reactions cease. The resultant molecular beam was probed by photoionization mass spectroscopy and IR spectroscopy. → Ethanethiol was found to undergo unimolecular decomposition by three pathways: CH3CH2SH fi (1) CH3CH2 + SH, (2) CH3 +H2C S, and (3) H2C CH2 +H2S. The experimental ndings are in good agreement with electronic structure calculations. 1. INTRODUCTION reaction pathway in which the C−S bond is cleaved. However, Raw energy sources including coal, petroleum, and biomass Baldrige et al. predicts that at higher temperatures (above 1000 K) the molecular elimination channels will be competitive with contain varying quantities of sulfur contaminants. Converting radical processes.10 raw energy sources into usable liquid fuels is a complicated The thermal pyrolysis of ethanethiol shown in (1) has been process involving many steps, but two aspects are common to all studied in static reactors, flow reactors, and flames over the last methods: (1) the use of heat to break chemical bonds of larger − 80 years.1 4 organic molecules into smaller semivolatile or volatile species fi and (2) the need to remove sulfur during the re ning process. CH32 CH SH+Δ→ Products (1) The thermolysis chemistry of the sulfur compounds encountered Our present view on the thermal cracking of ethanethiol, in petroleum and biofuels is poorly understood and, in some 2 cases, completely unknown. The current foundations in this field CH3CH2SH, is based on the early analysis of Sehon et al. who are based on end point chemistry, meaning that the reaction suggested two thermal decomposition channels. The lower temperature pathway (785−938 K) (2) proceeds by molecular mechanisms have been proposed without direct evidence of fi radical intermediates. This knowledge gap hinders any progress rearrangement to form ethylene (C2H4) and hydrogen sul de in refinery cleanup methodology, as current efforts for improving (H2S) sulfur removal technologies are done with an incomplete picture CH32 CH SH→+ C 242 H H S (2) of the molecular level chemistry. − Despite the importance of sulfur in industrial processing, there and, at high temperatures, by a free radical (1005 1102 K) has been very little attention to the pyrolysis of organic sulfur mechanism (3) to form the ethyl radical (C2H5) and sulfanyl (or compounds. The thermal chemistry of ethanethiol has been mercaptal) radical (SH). studied sporadically over the years, which has resulted in several − CH32 CH SH→+ C 25 H SH (3) proposed thermal decomposition mechanisms.1 4 Previous work focused on the disappearance of the parent thiol with little The compounds observed by Sehon et al. were H2S, C2H4,H2, attention, as a result of experimental limitations, to the and CH4. Due to the experimental limitations of Sehon et al. as identification and characterization of thermal decomposition well as other previous studies, reactive intermediates (i.e., SH products. The lack of experimental information on initial radical in (3)) produced as pyrolysis products were unable to be decomposition products has led to disagreement on the reaction detected. For this reason, the current mechanism for the thermal − mechanisms for ethanethiol and many other alkanethiols.5 9 decomposition of ethanethiol is based on end point chemistry Sehon et al.2 and Bamkole et al.5 proposed a radical without direct evidence of radical intermediates. As is the case mechanism for ethanethiol pyrolysis initiated by the cleavage with many molecules including ethanethiol, when end point of the C−S bond. Sehon et al. also suggested a second possible chemistry alone is used to propose thermal decomposition, mechanism along with Thompson et al. that proceeds by mechanisms often the most rich and unexpected chemistry is molecular elimination reactions.8,9 Recent theoretical work by overlooked. Baldridge et al.10 predicts that the molecular elimination channels for alkanethiols are unfavorable at low temperatures Received: March 20, 2017 (below 1000 K) as a result of the relatively high reaction barriers Revised: May 20, 2017 compared with that of the more energetically favored radical Published: May 30, 2017 © 2017 American Chemical Society 4953 DOI: 10.1021/acs.jpca.7b02629 J. Phys. Chem. A 2017, 121, 4953−4960 The Journal of Physical Chemistry A Article To understand reaction 1, it is important to examine the thermochemistry of ethanethiol as this will provide a powerful basis for the initial unimolecular decomposition channels. The thermochemistry for ethanethiol is well-known.11 The two − weakest bonds in ethanethiol are the C−S bond,12 14 − ± −1 − DH298K(CH3CH2 SH) = 73.6 0.5 kcal mol , and the C C 13 − ± −1 bond, DH298K(CH3 CH2SH) = 82.5 2.2 kcal mol . On the basis of thermochemistry, it is expected that unimolecular decomposition chemistry will be initiated by the cleavage of these two bonds. In this study, the thermal decomposition of ethanethiol was investigated using a pulsed microtubular reactor and two different methods of detection. The products of the reaction were monitored using 118.2 nm (10.487 eV) vacuum ultraviolet photoionization mass spectrometry (PIMS) and matrix isolation IR spectroscopy. Within the microtubular reactor, ethanethiol seeded in an inert carrier gas is rapidly heated to 1000−1300 K, Figure 1. Schematic of the pulsed microtubular reactor used for and decomposition is initiated. The relatively short residence ethanethiol thermal cracking. Samples were collected in a 5 K cryogenic time in the reactor (50−100 μs) ensures that the observed matrix for analysis by infrared absorption spectroscopy or photoionized chemistry emphasizes the unimolecular processes excluding all with fixed frequency vacuum ultraviolet (VUV) light with ion detection but the most rapid bimolecular chemistry. by a time-of-flight mass spectrometer. 2. EXPERIMENTAL SECTION matrix isolation apparatus is that it allows for accumulation of A high-temperature pulsed microtubular reactor (or hyper- nascent thermal decomposition products over time, promoting thermal nozzle) was used to decompose ethanethiol. The good signal-to-noise, while simultaneously isolating the nascent microtubular reactor is a version of the Chen−Ellison reactor products from each other within the matrix to prevent any that has been used for several years to produce reactive secondary reactions. − intermediates.15 26 The hyperthermal nozzle features a (1 mm The photoionization time-of-flight mass spectrometer that was i.d. × 3 cm long) SiC tube that can be heated up to 1700 K, with used in these experiments has been described in more detail 35,36 the temperature monitored by a type C thermocouple mounted elsewhere; therefore, it is only briefly discussed here. In to the outer wall of the SiC tube. A benefit of the microtubular contrast to the matrix isolation setup, as the nascent cracking reactor is the short residence time (50−100 μs).27 This short products exit the microtubular reactor, the supersonic jet is residence time eliminates problems one might face in a typical skimmed approximately 3−5 mm after exiting the SiC tube and vacuum pyrolysis experiment, including secondary reac- intersects with a 10.487 eV (118.2 nm) laser beam. The 10.49 eV − tions.28 34 light is generated by the frequency tripling of the third harmonic Thermal cracking products are produced by pulsing (356.6 nm, 30 mJ/pulse) of a Nd:YAG. The new ions are injected − fl fl ethanethiol, CH3CH2SH, seeded in an inert gas (roughly 1 2 by a positively biased repeller plate into a re ection time-of- ight atm) through the resistively heated SiC tube into a vacuum tube and accelerated into the drift zone by a strong electric field. chamber. The valve can be fired at a nominal rate of 10−50 Hz. At the end of the flight tube, the ions are reflected back down to The ethanethiol/Ar (≤0.1%) mixture is injected into the SiC the microchannel plate detector biased at negative voltage. The tube where it undergoes thermal decomposition and expands PIMS provides mass-to-charge (m/z) information that comple- supersonically into a vacuum chamber (≤10−6 Torr). The ments the vibrational frequencies obtained via the matrix rotational, vibrational, and translational temperatures drop isolation experiment. The PIMS, unlike the matrix experiment, rapidly within a few reactor diameters. The dynamics of the does not require an accumulation of products for detection, and pyrolysis and gas transport in the reactor has been well therefore, the time duration for an experiment is shorter. A characterized using computational fluid dynamics and will not typical matrix experiment can take up to 4 h for a single be discussed here.27 Two independent spectroscopic techniques, temperature, whereas the PIMS experiments can accomplish matrix isolation IR spectroscopy and PIMS, are used to monitor several temperatures in 1 h. For this reason, the PIMS was used and characterize the output of the microreactor. Figure 1 for quick determination of the optimal temperature for the provides a schematic of the experimental apparatus. thermal decomposition of ethanethiol. Matrix isolation infrared spectroscopy involves freezing the gas that is to be analyzed to a cold CsI window and then probing with 3. RESULTS: THERMAL DECOMPOSITION OF infrared light.